I think the .243, in general, is one of the most superior cartridges today. The .243 should be a viable choice for the "next" U.S. military rifle configuration, after they take a good, long look at the .223 and see its slight disadvantage (for combat, low fragmentation range, for competition, too much wind).
But yeah, who wouldn't want a caliber that has 50% more weight than the M193 bullet, at the SAME velocity?? Sure the recoil would be a bit stiffer, but as has already been unofficially confirmed, that of the .223 AR15 is not exactly a shoulder-bruiser. When I have some money to throw around in the next few years, I'll probably cough up the money for an AR-10 A2.
What exactly is the 6mm? It's a very popular Highpower Match round, is it as flat/fast as the .243? What original cartridge specification is it "derived" from, like a necked-down 7mm or something?